Neurological diseases are associated with the death of or injury to neuronal cells. For example, the loss of dopaminergic neurons in the substantia nigra is the basis for Parkinson's disease. Although the molecular mechanism of neurodegeneration in Alzheimer's disease is yet to be established, inflammation and deposition of beta-amyloid protein and other such agents may compromise neuronal function or survival. In patients suffering from brain ischemia or spinal cord injuries, extensive neuronal cell death is observed. Currently, there are no satisfactory treatments for these diseases.
One approach to treating neurological diseases involves the use of drugs capable of inhibiting neuronal cell death. A more recent approach involves the promotion of nerve regeneration by drugs which stimulate neurite outgrowth.
Neurite outgrowth may be stimulated in vitro by nerve growth factors, such as NGF. For example, Glial Cell Line-Derived Neurotrophic Factor (GDNF) demonstrates neurotrophic activity both, in vivo and in vitro, and is currently being investigated for the treatment of Parkinson's disease. Insulin and Insulin-like growth factors have been shown to stimulate growth of neurites in rat pheochromocytoma PC12 cells and in cultured sympathetic and sensory neurons [Recio-Pinto et al., J. Neurosci., 6, pp. 1211-1219 (1986)]. Insulin and Insulin-like growth factors also stimulate the regeneration of injured motor nerves in vivo and in vitro [Near et al., PNAS, pp. 89, 11716-11720 (1992); and Edbladh et al., Brain Res., 641, pp. 76-82 (1994)]. Similarly, fibroblast growth factor (FGF) stimulates neural proliferation [D. Gospodarowicz et al., Cell Differ., 19, p. 1 (1986)] and growth [M. A. Walter et al., Lymphokine Cytokine Res., 12, p. 135 (1993)].
There are, however, several disadvantages associated with the use of nerve growth factors for treating neurological diseases. They do not readily cross the blood-brain barrier. They are unstable in plasma. And they have poor drug delivery properties.
Recently, small molecules have been shown to stimulate axonal outgrowth in vivo. In individuals suffering from a neurological disease, this stimulation of neurite outgrowth may protect neurons from further degeneration, and accelerate the regeneration of nerve cells. For example, estrogen has been shown to promote the growth of axons and dendrites, which are neurites sent out by nerve cells to communicate with each other in a developing or injured adult brain [(C. Dominique Toran-Allerand et al., J. Steroid Biochem. Mol. Biol., 56, pp. 169-78 (1996); and B. S. McEwen et al., Brain Res. Dev. Brain. Res., 87, pp. 91-95 (1995)]. The progress of Alzheimer's disease may be slowed in women who take estrogen. Estrogen is hypothesized to complement NGF and other neurotrophins and thereby help neurons differentiate and survive.
It has been reported that compounds with an affinity for the FK506 binding protein (FKBP) that inhibit that protein's rotamase activity also possess nerve growth stimulatory activity. [Lyons et al., PNAS, 91, pp. 3191-3195 (1994)]. Many of these such compounds also have immunosuppressive activity.
FK506 (Tacrolimus), an immunosuppressive drug, has been demonstrated to act synergistically with NGF in stimulating neurite outgrowth in PC12 cells as well as sensory ganglia [Lyons et al. (1994)]. This compound has also been shown to be neuroprotective in focal cerebral ischemia [J. Sharkey and S. P. Butcher, Nature, 371, pp. 336-339 (1994)] and to increase the rate of axonal regeneration in injured sciatic nerve [B. Gold et al., J. Neurosci., 15, pp. 7509-16 (1995)].
The use of immunosuppressive compounds, however, has obvious drawbacks. In addition to compromising immune function, prolonged treatment with these compounds can cause nephrotoxicity [Kopp et al., J. Am. Soc. Nephrol., 1, p. 162 (1991)], neurological deficits [P.C. DeGroen et al., N. Eng. J. Med., 317, p. 861 (1987)] and vascular hypertension [Kahan et al., N. Eng. J. Med., 321, p. 1725 (1989)].
More recently, sub-classes of FKBP binding compounds which inhibit rotamase activity, but which purportedly lack immunosuppressive activity have been disclosed for use in stimulating nerve growth [see U.S. Pat. Nos. 5,614,547; 5,696,135; WO 96/40633; WO 96/40140; WO 97/16190; J. P. Steiner et al., Proc. Natl. Acad. Sci. USA , 94, pp. 2019-23 (1997); and G. S. Hamilton et al., Bioorg. Med. Chem. Lett., 7, pp. 1785-90 (1997)]. While these compounds supposedly avoid certain unwanted side effects of immunosuppressive FKBP binding compounds, they still bind to FKBP and inhibit its rotamase activity. This latter property may still lead to undesirable side effects due to other roles FKBP may play in mammals.
Surprisingly, it is now known that binding to FKBP is not necessary for neuronal activity. Co-pending U.S. patent application Ser. Nos. 08/748,447, 08/748,448 and 08/749,114 each describe the use of non-FKBP binding, non-immunosuppressive compounds for stimulating nerve growth and preventing neurodegeneration. Due to their lack of affinity for FKBP, these compounds advantageously avoid any potential interference with FKBP-associated biochemical pathways. These compounds do, however, inhibit multi-drug resistance ("MDR") through inhibition of the p-glycoprotein and MRP. While it appears that the dosages of those compounds necessary to stimulate nerve growth and prevent neurodegeneration are lower than those that effect MDR, it would still be desirable to obtain compounds which are specific for neuronal activity, without other significant mechanisms of action.
Though a wide variety of neurological degenerative disorders may be treated by stimulating neurite outgrowth, there are relatively few agents known to possess these properties. Moreover, the newer non-immunosuppressive compounds have only recently begun being tested in living organisms. Thus, there remains a need for new pharmaceutically acceptable compounds and compositions that have the ability to stimulate neurite outgrowth and prevent neurodegeneration in patients without causing immunosuppression, without interfering with FKBP and without affecting cellular pumps, such as p-glycoprotein or MRP.